Alacritty_terminal: Spawned Process Not Returning Error Code When Segfault Occurs

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Introduction

In this article, we will delve into an issue encountered while using the alacritty_terminal library to spawn processes into a terminal. The problem arises when a spawned process crashes due to a segmentation fault, resulting in the event loop receiving a tty::ChildEvent::Exited event with no error code. We will explore the code responsible for handling this event and discuss possible reasons why the expected error code is not being returned.

Spawned Process and Event Loop

The alacritty_terminal library is designed to spawn processes into a terminal, allowing for seamless interaction between the spawned process and the terminal. In this instance, we are spawning a process using the pty module, which provides a pseudo-terminal for the spawned process to interact with.

let pty_config = tty::Options {
    shell: Some(tty::Shell::new(settings.executable, settings.args)),
    working_directory: settings.working_directory,
    ..tty::Options::default()
};

The event loop is responsible for monitoring the spawned process and handling events such as process exit. When the process exits, the event loop receives a tty::ChildEvent::Exited event, which contains the error code of the process.

// alacritty_terminal/src/event_loop.rs 259
if let Some(tty::ChildEvent::Exited(code)) = self.pty.next_child_event()
{
    println!("GOT EXIT EVENT. Code: {:?}", code);
    if let Some(code) = code {
        println!("Calling childexit");
        self.event_proxy.send_event(Event::ChildExit(code));
    }
    if self.drain_on_exit {
        let _ = self.pty_read(&mut state, &mut buf, pipe.as_mut());
    }
    println!("Calling terminal.exit");
    self.terminal.lock().exit();
    self.event_proxy.send_event(Event::Wakeup);
    break 'event_loop;
}

Expected Behavior

When a process crashes due to a segmentation fault, we would expect the error code to be returned as 139. This is because the operating system typically returns a non-zero error code when a process terminates abnormally.

python will_crash.py
echo $?

However, in our case, the event loop is receiving a tty::ChildEvent::Exited event with no error code.

Possible Reasons

There are several possible reasons why the expected error code is not being returned:

  1. pty Module: The pty module may not be correctly handling the process exit event, resulting in the error code not being returned.
  2. Event Loop: The event loop may not be properly configured to handle process exit events, leading to the error code not being returned.
  3. Operating System: The operating system may not be returning the expected error code when a process crashes due to a segmentation fault.

Conclusion

In conclusion, the issue encountered with the alacritty_terminal library is likely due to a combination of factors, including the pty module, event loop configuration, and operating system behavior. To resolve this issue, we need to investigate each of these factors and the necessary adjustments to ensure that the expected error code is returned when a process crashes due to a segmentation fault.

Code Review

Let's take a closer look at the code responsible for handling the process exit event.

// alacritty_terminal/src/event_loop.rs 259
if let Some(tty::ChildEvent::Exited(code)) = self.pty.next_child_event()
{
    println!("GOT EXIT EVENT. Code: {:?}", code);
    if let Some(code) = code {
        println!("Calling childexit");
        self.event_proxy.send_event(Event::ChildExit(code));
    }
    if self.drain_on_exit {
        let _ = self.pty_read(&mut state, &mut buf, pipe.as_mut());
    }
    println!("Calling terminal.exit");
    self.terminal.lock().exit();
    self.event_proxy.send_event(Event::Wakeup);
    break 'event_loop;
}

Upon reviewing the code, we notice that the pty module is not correctly handling the process exit event. The pty module is responsible for monitoring the spawned process and returning the error code when the process exits. However, in this instance, the pty module is not returning the expected error code.

Possible Fixes

To resolve this issue, we can try the following fixes:

  1. Update pty Module: Update the pty module to correctly handle process exit events and return the expected error code.
  2. Configure Event Loop: Configure the event loop to properly handle process exit events and return the expected error code.
  3. Modify Operating System Behavior: Modify the operating system behavior to return the expected error code when a process crashes due to a segmentation fault.

Future Work

In the future, we can explore the following areas to improve the alacritty_terminal library:

  1. Improve pty Module: Improve the pty module to handle process exit events more efficiently and return the expected error code.
  2. Enhance Event Loop: Enhance the event loop to properly handle process exit events and return the expected error code.
  3. Optimize Operating System Behavior: Optimize the operating system behavior to return the expected error code when a process crashes due to a segmentation fault.

Introduction

In our previous article, we explored the issue encountered with the alacritty_terminal library when spawning processes into a terminal. The problem arises when a spawned process crashes due to a segmentation fault, resulting in the event loop receiving a tty::ChildEvent::Exited event with no error code. In this article, we will provide a Q&A section to address common questions and concerns related to this issue.

Q: What is the expected behavior when a process crashes due to a segmentation fault?

A: When a process crashes due to a segmentation fault, the operating system typically returns a non-zero error code, usually 139. This error code indicates that the process terminated abnormally.

Q: Why is the pty module not returning the expected error code?

A: The pty module may not be correctly handling the process exit event, resulting in the error code not being returned. This could be due to a bug in the pty module or incorrect configuration of the event loop.

Q: How can I configure the event loop to properly handle process exit events?

A: To configure the event loop to properly handle process exit events, you can try the following:

  1. Update the pty module to correctly handle process exit events and return the expected error code.
  2. Configure the event loop to properly handle process exit events and return the expected error code.
  3. Modify the operating system behavior to return the expected error code when a process crashes due to a segmentation fault.

Q: What are the possible reasons why the expected error code is not being returned?

A: There are several possible reasons why the expected error code is not being returned:

  1. pty Module: The pty module may not be correctly handling the process exit event, resulting in the error code not being returned.
  2. Event Loop: The event loop may not be properly configured to handle process exit events, leading to the error code not being returned.
  3. Operating System: The operating system may not be returning the expected error code when a process crashes due to a segmentation fault.

Q: How can I troubleshoot this issue?

A: To troubleshoot this issue, you can try the following:

  1. Check the pty module to ensure it is correctly handling process exit events.
  2. Verify that the event loop is properly configured to handle process exit events.
  3. Investigate the operating system behavior to determine why the expected error code is not being returned.

Q: What are the possible fixes for this issue?

A: To resolve this issue, you can try the following fixes:

  1. Update the pty module to correctly handle process exit events and return the expected error code.
  2. Configure the event loop to properly handle process exit events and return the expected error code.
  3. Modify the operating system behavior to return the expected error code when a process crashes due to a segmentation fault.

Q: What are the future work areas for improving the alacritty_terminal library?

A: In the future, we can explore the following areas to improve the alacritty_terminal library:

  1. Improve pty Module: Improve the pty module to handle process exit events more efficiently and return the expected error code.
  2. Enhance Event Loop: Enhance the event loop to properly handle process exit events and return the expected error code.
  3. Optimize Operating System Behavior: Optimize the operating system behavior to return the expected error code when a process crashes due to a segmentation fault.

By addressing these areas, we can improve the alacritty_terminal library and ensure that it behaves as expected when spawning processes into a terminal.